1. Field of the Invention
The present invention relates to an optical wavelength division multiplexed signal monitoring apparatus.
2. Description of the Related Art
Wavelength division multiplexing (WDM) is a technique that multiplexes a plurality of optical signal channels (called WDM channels from now on) with different carrier optical wavelengths into a single optical fiber and transmits them through the optical fiber. The technique is useful to deal with an increasing transmission capacity. The WDM technique includes the following as typical signal monitoring apparatuses.
Conventional technique 1: It identifies a faulty section and obtains a switching start signal for each WDM channel by carrying out parity check called bit interleaved parity between repeaters or multiplexing terminals by using overhead specified in the synchronous optical network (SONET)/synchronous digital hierarchy (SDH) transmission scheme (reference material [1]: ITU-T Recommendation G.707).
Conventional technique 2: It observes an optical spectrum, and measures signal quality degradation for each WDM channel by monitoring an optical signal-to-noise ratio.
FIG. 1 shows a configuration of a wavelength division multiplex signal monitoring apparatus of the conventional technique 1. The conventional signal monitoring apparatus comprises an optical wavelength division demultiplexer 61 for carrying out optical wavelength division demultiplexing of an optical wavelength division multiplexed signal consisting of N optical signals with a bit rate f0 (bits/s) which are wavelength multiplexed, (where N is an integer greater than one); and N electric signal processors 62 for processing N-channel optical wavelength division demultiplexed signals which are demultiplexed by the optical wavelength division demultiplexer 61. Each electric signal processor 62 includes a photoelectric converter (receiving circuit) 63, a clock extracting section (clock extracting circuit) 64 and an error detecting section 65 consisting of a parity check circuit or a comparing circuit. With an increase in the signal bit rate or variety of the signal formats for each WDM channel, the conventional technique 1 requires an increasing number of receiving systems (electric signal processors 62) suitable for the bit rate, signal format or modulation method (NRZ (Non Return to Zero) or RZ (Return to Zero)) of each signal. In addition, when the number of the WDM channels increases by a factor of N, N receiving systems are required for each of them, thereby increasing the scale of the apparatus tremendously.
FIG. 2 shows a configuration of a wavelength division multiplex signal monitoring apparatus of the conventional technique 2. The conventional signal monitoring apparatus comprises an optical spectrum analyzer 62-1 for observing the optical spectrum of an optical wavelength division demultiplexed singal, and for measuring the signal quality degradation in each WDM channel by monitoring the optical signal-to-noise ratio. Although the conventional technique 2 can obtain the optical signal-to-noise ratio, it has a problem in that it cannot detect the waveform degradation due to the wavelength dispersion in an optical fiber, or the transmission degradation due to the waveform degradation by the polarization dispersion, and that it cannot reflect the bit error rate directly.
FIG. 3 shows a configuration of a conventional example 3 of a wavelength division multiplex signal monitoring apparatus. The conventional signal monitoring apparatus comprises a photoelectric converter 63 for converting a single-wavelength optical wavelength division demultiplexed singal into an electric intensity modulated signal; a sampling clock generator 66 for generating a sampling clock signal with a repetition frequency of f1 (Hz)=(n/m)f0+a, where n and m are a natural number and a is an offset frequency; and an electric signal processor 67. The electric signal processor 67 samples the electric intensity modulated signal output from the photoelectric converter 63 by the sampling clock signal the sampling clock generator 66 generates, obtains optical signal intensity distribution from the sampled signal, and evaluates an optical signal quality parameter on the basis of the optical signal intensity distribution.
FIG.4 shows a configuration of an example 4 of the conventional wavelength division multiplex signal monitoring apparatus. The conventional signal monitoring apparatus comprises an optical sampling pulse train generator 68; an optical multiplexer 69; a nonlinear optical medium 70; an optical splitter 71; a photoelectric converter 72; and an electric signal processor 73. The optical sampling pulse train generator 68 generates an optical sampling pulse train, the repetition frequency of which is f1=(n/m)f0+a, where n and m are a natural number and a is an of offset frequency. The pulse width of the pulses of the optical sampling pulse train is much narrower than the time slot of an optical signal with a bit rate f0 (bits/s). The optical multiplexer 69 combines the optical wavelength division demultiplexed singal of a certain wavelength and the optical sampling pulse train generated by the optical sampling pulse train generator 68. The nonlinear optical medium 70 induces nonlinear interaction between the optical wavelength division demultiplexed singal and the optical sampling pulse train, which are combined by the optical multiplexer 69. The optical splitter 71 splits a cross-correlation optical signal, which is brought about by the nonlinear interaction in the nonlinear optical medium 70, from the optical wavelength division demultiplexed singal or from the optical sampling pulse train. The photoelectric converter 72, receiving the cross-correlation optical signal the optical splitter 71 outputs, converts it into the electric intensity modulated signal. The electric signal processor 73 calculates the optical signal intensity distribution from the electric intensity modulated signal supplied from the photoelectric converter 72, and evaluates the optical signal quality parameter on the basis of the optical signal intensity distribution.
The conventional examples as shown in FIGS. 3 and 4 are a method that evaluates the optical signal quality parameter from the amplitude histogram (reference material [2]: EPC publication No. EP0920150A2, U.S. patent application Ser. No. 09/204,001 which is not yet laid-open). Although they can respond to an increase in the signal bit rate and an increase in the number of signal formats flexibly, and monitor the optical signal degradation such as waveform degradation due to the wavelength dispersion in the optical fiber, they are not applicable to a multi-wavelength optical signal.
FIG.5 shows a configuration of an example 5 of the conventional wavelength division multiplex signal monitoring apparatus. It consists of the configurations of FIG.3, which are connected in parallel by the number of the WDM channels using an optical wavelength division demultiplexer 74. FIG.6 shows a configuration of an example 6 of the conventional wavelength division multiplex signal monitoring apparatus. As the example 5, it consists of the configurations of FIG.4, which are connected in parallel by the number of the WDM channels using an optical wavelength division demultiplexer 78.
The conventional examples 5 and 6 consist of the configurations of the conventional examples 3 and 4 in parallel by the number of the WDM channels. Accordingly, they have a problem in that when the number of the WDM channels is N, the scale of the apparatus increases by a factor of N.
On the other hand, the multimedia service market has boomed in recent years, and communication capacity of individual services must be increased. In addition, networks are required that satisfy a variety of signal bit rates and signal formats for the video, sound and data. Thus, an optical wavelength division multiplexed signal monitoring apparatus is required that can respond flexibly to an increase in the number of the WDM channels, an increase in the signal bit rate per WDM channel, and diversification of the signal format. In connection with this, the optical signal degradation factors to be monitored are also diversified. In particular, it is necessary to monitor the waveform degradation involved in the wavelength dispersion in the optical fiber, and the waveform degradation due to the polarization dispersion.
Therefore, an optical wavelength division multiplexed signal monitoring apparatus is eagerly required that can monitor the optical signal degradation factors such as waveform degradation involved in the wavelength dispersion in the optical fiber, and can respond flexibly to an increase in the number of the WDM channels, an increase in the signal bit rate of each WDM channel, and the diversification of the signal formats.